Every push to main now builds the release binary once and stores it in the
actions cache as rustfs-baseline-<sha> (build-baseline-cache job). The A/B job
restores it for origin/main and passes --baseline-bin; on the nightly, where
the candidate commit equals the baseline, one cached binary serves both phases
with --skip-build and the run does zero source builds. A cache miss falls back
to the source double-build via --baseline-ref origin/main.
This removes the ~32min-per-side double build that pushed the 24-cell nightly
past its 120min ceiling (2026-07-11..07-14 all cancelled on timeout).
Also:
- alert-on-failure now fires on cancelled/timed-out, not just failure, so a
timed-out nightly is no longer silent (the composite action already reports
cancelled jobs); removed the stale perf-2 TODO now that the alert job exists.
- run_hotpath_warp_ab.sh appends a Provenance section to gate.md (baseline and
candidate SHAs + binary source, runner, warp version, matrix params) via a
repeatable --provenance-note; the gate exit code is preserved.
Refs rustfs/backlog#1152 (perf-3).
